Why are there holes in my exported meshes?
The faces on the sides of the knife seem to be N-gons.
Try triangulating the mesh before exporting it:

In edit mode (Tab), select everything (A) or just the ngons by pressing 3D view &gt; Header &gt; Select &gt; Faces by sides, then setting the type to greater than in the redo panel (F6):


Triangulate the faces by pressing CtrlT:



If you want more control over the triangulation, you might try using the triangulate modifier.